I read through this whole thing with trepidation. I fear of being that parent. At the moment I am 77 and despite some physical limitations and the need to sometimes hunt for a word I've known all my life, I still have my cognitive faculties. I do not want my children to go through any kind of hardship other than the inevital grief.
I have a large family who lives with me, a daughter, 3 of her 6 kids, their spouses/others and a couple great grandkids. They take care of me now, ands insist they will always do so; they want me to live forever.
But I know with each passing year I need more and more help to do things, and I hate this. There will never be money for facilitated living, and I would never want that anyway.
So I am grateful for my family.
I feel for you and the nightmare you have been going through, and if your parents are at all aware, I'm sure they are grateful. I am happy they have gotten to live out their days together.
Try to take some time for yurself now, and rebuild your strength and zest for your own life.